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Raman scattering of (K0.5Na0.5)NbO3-0.05LiNbO3-yMn02 (y = 0% and 1.0%) single crystals have been reported in the temperature range from 300 to 800 K. The spectra exhibit a competition between a soft mode and a relaxation mode upon heating across the diverse transitions. The progressive change in the conflicting displacive mechanism (soft mode) and order-disorder (relaxation mode) can explain the origin of the successive orthorhombic-tetragonal-cubic phase transitions. Moreover, the polymorphic phase transition between orthorhombic and tetragonal structures can be confirmed through the observation of thermotropic phase boundaries for MnO2-doped (K0.5Na0.5)NbO3-0.05LiNbO3 single crystals. [ABSTRACT FROM AUTHOR]

Optical properties and phase transitions of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) crystals near morphotropic phase boundary (MPB) have been investigated by temperature dependent transmittance and reflectance spectra. Three critical point energies E8=3.17-3.18 eV, Ea=3.41-3.61 eV, and Eb = 4.74-4.81 eV can be assigned to the transitions from oxygen 2p to titanium d, niobium d, and lead 6p states, respectively. They show narrowing trends with increasing temperature, which can be caused by thermal expansion of the lattice and electron-phonon interaction. Deviation from the linear behaviors can be observed from Ea and Eb versus PT concentration, indicating a complex multiphase structure near MPB region. [ABSTRACT FROM AUTHOR]

Optical properties of Na0.5Bi0.5TiO3-6%BaTiO3 (NBT-6%BT) single crystal have been studied by temperature dependent Raman and ultraviolet-visible spectra from 25 to 180 °C. With increasing the temperature, the absorption edge approximately decreases from 3.13 to 3.04 eV. Moreover, abnormal changes of phonon mode and spectral transmission are observed at 83, 106, and 150 °C, which can be unambiguously correlated with thermal evolutions of polar nano-regions and phase transition. It indicates that there is an inherent relationship between optical behavior and structural variation of NBT-6%BT, which provides a valid methodology to explore the phase transition of relaxor ferroelectric oxides. [ABSTRACT FROM AUTHOR]

We report band to band transition behaviors of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals derived from temperature-dependent spectral transmittance. A typical bandgap formula with the temperature and composition (8 K≤Texp≤453 K, 0.1≤x≤0.4) has been presented. Moreover, the phase diagram of PMN-xPT crystals can be well proposed, which is based on the bandgap variations and can be explained by electronic structure evolution. It reveals an intrinsic relationship between fundamental bandgap and phase transition of PMN-xPT single crystals, which pioneers an effective methodology to explore the phase transition of ferroelectric oxides. [ABSTRACT FROM AUTHOR]

The transmittance spectra of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (x=0.24 and 0.31) single crystals have been studied in the temperature range of 5.3-300 K. It was found that the direct band gap Egd is 3.150±0.016 eV, indirect band gap Egi is 2.939±0.014 eV, and the phonon energy Ep is 0.098±0.014 eV for the PMN-0.24PT crystal at 300 K. With increasing the temperature, the Egd of the PMN-0.24PT crystal decreases from 3.263±0.017 to 3.150±0.016 eV while the Egd of the PMN-0.31PT crystal increases from 3.050±0.015 to 3.101±0.016 eV. The peculiar characteristic can be ascribed to the monoclinic and rhombohedral multiphase coexistence in the PMN-0.31PT crystal. [ABSTRACT FROM AUTHOR]